This is a video of a common moorhen.
From Wildlife Extra:
One of our readers has sent us some fantastic photos of a Leucistic moorhen taken in her local park. The moorhen was one of a brood, but none of the others birds show any signs of leucism.
Leucism (or Leukism)
Leucism is a very unusual condition whereby the pigmentation cells in an animal or bird fail to develop properly. This can result in unusual white patches appearing on the animal, or, more rarely, completely white creatures.
Greater spotted woodpecker, with orange rump: here.
White ‘Leucistic’ Buzzard Seen In Austria: here.
Leucistic jackdaw photos: here.
Leucistic Great tit: here.
Leucistic North American birds: here.